The present invention relates to an apparatus for reading from and/or writing to optical recording media by means of a scanner, the apparatus having a measuring device for determining the travel of a scanner.
An apparatus of this type is disclosed in U.S. Pat. No. 4,977,539. The measuring device of this apparatus has a rotary disc provided with slots and a photosensor, which is used to detect the rotation of the rotary disk. The rotary disk is connected to a motor which drives the scanner. The known apparatus may be regarded as having the disadvantage that hysteresis occurs inter alia on account of the mechanical linking of the rotary disk to the motor and the geometrical conditions. It is not possible, therefore, to accurately determine the position of the scanner.
The object of the present invention is to propose an apparatus in which it is possible to determine the position of the scanner as exactly as possible, the intention being for this to be ensured even in the event of a high movement speed or acceleration.
To that end, the invention provides for the measuring device to have an optical interference generating means and also an optical interference detection means. According to the invention, the position of the scanner is determined by means of optical interference, which has the advantage that a high measurement accuracy is attained. A further advantage resides in the fact that no hysteresis occurs, as a result of which the high measurement accuracy is ensured even in the start-up and deceleration phases and at high speeds. The apparatus according to the invention therefore ensures rapid and exact access to data stored on the recording medium, irrespective of whether they are arranged contiguously or distributed at extremely diverse locations on the recording medium.
It is advantageous for the interference generating means or the interference detection means to be arranged on the scanner, while the corresponding other one is arranged such that it is immovable. If the optical interference generating means is arranged on the scanner and the interference detection means is arranged on a baseplate with respect to which the scanner can be displaced, this has the advantage that optical elements that are present on the scanner can also be utilized for the interference generating means and only a small number or no additional components are necessary. It is likewise advantageously possible to arrange the interference detection means on the scanner and the interference generating means on the baseplate. This has the advantage that the output signal of the interference detection means is directly available at the scanner, where other signals are also detected. Joint evaluation at the location of the scanner is thus made possible. So too is joint forwarding of the detected signals from the scanner to an evaluation unit, if appropriate after previous signal conditioning. This advantageously takes place on the scanner and extends from simple preamplification through to processing or combination of the individual signals.
According to a further refinement of the invention, only part of the optical interference generating means and/or of the optical interference detection means is arranged on the scanner and/or such that it is immovable. This has the advantage that the measuring device is arranged to the greatest possible extent on the same component, either on the scanner or immovably on the baseplate, while only part of the measuring device is arranged on the corresponding other part. This simplifies the adjustment and also the production of the measuring device. In an advantageous manner, just a mirror is arranged on the part remote from the rest of the measuring device, while the interference is effected for example by superposition of the reflected light beam with the output light beam.
A further refinement of the invention provides for the interference generating means to be arranged in the output beam path of an optical element having at least one further output beam path, in which another optical component utilized for the operation of the apparatus is arranged. This has the advantage that a previously unutilized output beam path is utilized for the purpose of generating interference. This means that an additional component for coupling out or generating the light utilized by the interference generating means is not necessary. In this case, the optical element may be a half-mirror or a beam splitter which serves for example for directing the beam reflected from the recording medium onto a detector element. An optical grating which generates possibly unutilized first- or higher-order output beams, if appropriate in the reflection direction, is also a possible configuration of the optical element. The optical recording medium itself, whose grating structure formed by tracks likewise generates first- and higher-order output beams, may also constitute the said optical element.
The invention provides for the interference generating means to have a prism. This has the advantage that part of the beam falling onto the interference generating means is deflected by means of the prism and this deflected beam is superposed with the other part of the beam. In this way, a light-dark pattern is obtained in the beam propagation direction, which pattern is detected by the interference detection means.
According to the invention, the interference detection means is a photoelement which detects light-dark differences. This has the advantage that intensity fluctuations do not have a disturbing influence. A suitable evaluation circuit can match the photoelement to a changed intensity, in which case, by way of example, the threshold value utilized for discriminating between light and dark is put at the median between maximum and minimum of the photosignal. The output signal of the photoelement is an output value that merely discriminates between light and dark, that is to say a digital output value. The travel can be precisely determined by counting the transitions of this output signal.
The invention provides for the effective area of the photoelement to be limited to half the spacing between two interference maxima of the interference pattern generated by the interference generating means. This has the advantage of enabling a clear discrimination of light-dark transitions even with a relatively large and thus cost-effective photoelement. In this case, the limitation is advantageously effected by inclining the photoelement in the beam path. It is likewise advantageous to perform the limitation by fitting a masking in front of the photoelement.
An advantageous method for determining the travel of a scanner of an apparatus for reading from or writing to optical recording media is specified in the method claim. In this case, the fact that the count is multiplied by a constant factor may result from the characteristic of the interference pattern. However, more complicated functions are also conceivable which are characteristic of the corresponding interference pattern and according to which the travel is determined from the count. The utilization of a reference value table also lies within the scope of the method according to the invention.
It is understood that the features specified can also advantageously be applied within the scope of the invention in combinations other than those expressly described. This applies equally to developments which lie within the scope of expert ability. Further advantages of the invention are also specified in the following description of an advantageous exemplary embodiment with reference to the figures.